The issue of how many species inhabit our planet is a hot one: I’m often asked about it by journalists or in lectures, despite the fact that I have virtually no expertise in this area. We know that about 1.2 million species have been described, but we also know that that’s a gross underestimate of the true number. Any time somebody samples the insects in the rain forest, for example, a large number of new species turn up, and only Ceiling Cat knows how many species of nematodes and other worms live underground. And then there’s the real can of worms: the bacteria and archaea (see below). My usual answer to the question of “how many species?” is “probably between 5 and 50 million species.” Well, at least I was in the right range.

Here’s the authors’ conclusion, breaking down the number of species by group; I’ll then briefly describe how they calculated these numbers (my emphasis):

When applied to all eukaryote kingdoms, our approach predicted ~7.77 million species of animals, ~298,000 species of plants, ~611,000 species of fungi, ~36,400 species of protozoa, and ~27,500 species of chromists; in total the approach predicted that ~8.74 million species of eukaryotes exist on Earth (Table 2). Restricting this approach to marine taxa resulted in a prediction of 2.21 million eukaryote species in the world’s oceans.

This means that about 1.2/8.7, or roughly 14%, of all Earth’s species have been described. Note as well that about 89% of Earth’s species (7.77/8.74) are animals, and that the vast majority of those animals are insects (see second graph below). It was in fact Bob May who said, “To a first approximation, all animals are insects.”

How did they figure this out? By using a two-step process. First , they looked at “higher” taxa—phyla, classes, orders, family, and genera—and examined how the number of described taxa has increased over time, as well as the number of species. As you see from the graph below, which examines only animals (“Animalia”), every level above that of species is reaching an asymptote, while the number of species themselves continues to increase non-asymptotically.

Figure 1 from Mora et al. The caption: “Predicting the global number of species in Animalia from their higher taxonomy. (A–F) The temporal accumulation of taxa (black lines) and the frequency of the multimodel fits to all starting years selected (graded colors). The horizontal dashed lines indicate the consensus asymptotic number of taxa, and the horizontal grey area its consensus standard error. (G) Relationship between the consensus asymptotic number of higher taxa and the numerical hierarchy of each taxonomic rank. Black circles represent the consensus asymptotes, green circles the catalogued number of taxa, and the box at the species level indicates the 95% confidence interval around the predicted number of species (see Materials and Methods).”

The conclusion from these observations is that we’re starting to approach a complete knowledge (the asymptote) of the numbers and nature of “higher” taxa, but haven’t yet approached a full knowledge of the number of individual species in these groups.

The authors then made a clever hypothesis: perhaps there is a quantitative relationship between the number of higher taxa (whose final numbers we’re starting to approach), and the number of species contained within these higher taxa. If that were the case, we could use the asymptotic estimates for, say, number of families or orders, to estimate the final number of species they contain.

In fact, the authors did find such a quantitative relationship (see subfigure 1G above for a plot showing this): there appears to be a linear relationship on a log scale between taxonomic rank and number of asymptotic taxa. From this you can extrapolate to the taxon of interest on the graph—species—and get an estimate of the number of species on Earth, in this case for animals. But of course you can do this for other groups, too.

But is such extrapolation warranted? You can see whether it is by predicting from graph 1G above the number of species in well-studied groups (that is, groups, like birds and reptiles, in which we think we’ve already identified most of the species on Earth), and then comparing that prediction with the actual number of described species. The graph below shows that for groups that have been pretty exhaustively surveyed for species, the prediction holds pretty well: the predicted number using the higher-taxon extrapolation is pretty close to the actual number of known species:

Knowing that one can pretty accurately predict the number of species on Earth by extrapolating from the number of known higher taxa, the authors got their figures above simply by doing this kind of extrapolation in less well known groups, assuming that the same relationship holds for them. As I said, this is a clever idea, and it’s even endorsed by the critical Robert May in his accompanying perspective, “Why worry about how many species and their loss?”

Jonathan Eisen, a professor at the University of California at Davis, likes the paper as well, but criticizes part of it on his website, The Tree of Life, arguing (correctly, I think) that estimates for numbers of bacteria and archaea are way off because the “higher taxon” approach is invalid for those groups. If the estimates for these microbes really does run into the “hundreds of millions,” as Eisen thinks, then that would hugely inflate the number of species on Earth:

So all seems hunky dory and pretty interesting. That is, until we get to the bacteria and archaea. For example, check out Table 2. . . Their approach leads to an estimate of 455 ± 160 Archaea on Earth and 1 in the ocean. Yes, one in the ocean. Amazing. Completely silly too. Bacteria are a little better. An estimate of 9,680 ± 3,470 on Earth and 1,,320 ±436 in the oceans. Still completely silly. . . Their estimates of ~ 10,000 or so bacteria and archaea on the planet are so completely out of touch in my opinion that this calls into question the validity of their method for bacteria and archaea at all. Now you may ask – why do I think this is out of touch. Well because reasonable estimates are more on the order or millions or hundreds of millions, not tens of thousands.

So long will it take to catalog the rest of Earth’s biodiversity, and what would it cost? Fuggedaboutit: we don’t have the time, much less the bucks. As Mora et al. note:

Considering current rates of description of eukaryote species in the last 20 years (i.e., 6,200 species per year; +811 SD; Figure 3F–3J), the average number of new species described per taxonomist’s career (i.e., 24.8 species, [30]) and the estimated average cost to describe animal species (i.e., US$48,500 per species [30]) and assuming that these values remain constant and are general among taxonomic groups, describing Earth’s remaining species may take as long as 1,200 years and would require 303,000 taxonomists at an approximated cost of US$364 billion. With extinction rates now exceeding natural background rates by a factor of 100 to 1,000 [31], our results also suggest that this slow advance in the description of species will lead to species becoming extinct before we know they even existed. High rates of biodiversity loss provide an urgent incentive to increase our knowledge of Earth’s remaining species.

Bob May isn’t as pessimistic: he think that the bulk of Earth’s species could be discovered within a century of fairly intensive labor, although even then we’re going to miss many species since they’re rapidly going extinct. Mora et al. suggest where that labor should be concentrated:

. . .the bulk of species that remain to be discovered are likely to be small-ranged and perhaps concentrated in hotspots and less explored areas such as the deep sea and soil; although their small body-size and cryptic nature suggest that many could be found literally in our own ‘‘backyards’’ (after Hawksworth and Rossman [33]). Though remarkable efforts and progress have been made, a further closing of this knowledge gap will require a renewed interest in exploration and taxonomy by both researchers and funding agencies, and a continuing effort to catalogue existing biodiversity data in publicly available databases.

There are of course some potential problems with Mora et al.’s methods, and they do list them: they include uncertainty about how some taonomists define “species,” and the differential descriptive effort applied to different taxa. They also list problems with previous methods of estimating the number of species on Earth. I won’t go into this sort of detail since you can read the paper for free.

So why should we be cataloging all these species? That’s what Bob May’s perspective is about, and he gives three reasons. The first is because we can’t understand the evolutionary and ecological processes that create biodiversity unless we have a full understanding of the number of species. I’m not highly convinced by this, because you can understand those processes simply by studying a smaller sample. There simply aren’t an infinite number of ways that new species can form.

The other two reasons are practical. He argues that the number of species “underpins ecosystem services that . . humanity is dependent upon,” and that attempts to catalog biodiversity will invariably uncover some species that will be of tremendous aid to humanity. (He uses the example of a new variety of wild rice that, when crossed to domesticated rice, improves output by 30%. There is also the benefit for health, too: although May doesn’t mention this, a large proportion of the world’s pharmaceuticals ultimately trace back to compounds isolated from plants.)

For biologists like me, though, these economic considerations, while important, aren’t our real motivation. We simply want to know what’s out there, for among those millions of undescribed species are millions of curious and interesting tales: things to inspire not only wonder about nature, but research and new knowledge, which is valuable for its own sake. Humans are a species permeated with curiosity, and satisfying that curiosity is a good in itself. As I wrote in WEIT:

Each species represents millions of years of evolution, and, once gone, can never be brought back. And each is a book containing unique stories about the past. Losing any of them means losing part of life’s history.

Assuming that more species have gone extinct than have evolved in the last 4000 years, that must have been one crowded ark.

Thanks for expounding on this. The articles I’ve seen are either very short and superficial, or so technical it makes my head hurt. Once again, you have found a happy medium for us laymen who want to understand the methodology and implications. Have a great trip.

A agree. That series of asymptotic charts coupled with the simple extrapolation that the lowest order would follow the same pattern, coupled with confirmation…yeah, that nailed it down for me in a way that the articles in the popular press didn’t.

I hope we all ponder this estimates of the level of the biodiversity as the reminder of the complexity of the system that mankind as organism whole is yet to understand.

Scientists who learn about these topics through following their curiosity will eventually realize that they have no other choice but become not just “educators of the genarally ignborant public” but the personification of the inevitable process whereby “science” takes over “government”.

Our current socio-economic system is profoundly primitive. Our government (or better yet the lack of it) is fundamentally primitive and reflects beliefs of our evolutionary ignorant past.

I hope each of us as an indpendent thinker will spen a little time on what true sustainability really means and whether current socio-economic system can, in principle (the favorite of sam harris), be sustainable.

And then it is only one step towards understanding and contemplating the _evolution_ of “human condition” and the institutional changes we can exprect to see and be part of.

Full Disclosure: My Commentary may be incredibly biased since I know 4 of the authors, two of them quite well.

But I take a bit of issue with Jonathen Eisen’s criticism. Perhaps the paper could have worded stronger why the approach doesn’t hold for the Bacteria and Archea, but they do point out several factors that lead to the asymptotic prediction as being a lower bound. One of the reasons not explicitly mentioned is also just a lack of data. They chose to use two reference catalogs of described species (one for Eukaryotes and one for Prokaryotes) for their model and under those catalogs more stringent species description criteria there just aren’t as many described species of bacteria and Archea as there ought to be, which shows up particularly in the estimate for numbers of marine Archea. The other issue, only touched on in the paper but one that the authors are intimately familiar with is the whole issue of a species concept for population of microbial organisms that are probably sharing genes like crazy via LGT.

Thanks Jerry ~ from my layman, non-biologist perspective this is the best article I’ve read on the subject. It has stretched me to look more deeply – the word Chromista is new to me, even though it was coined 30 years ago, so I looked up Cavalier-Smith’s six kingdoms & I noted with dismay that it doesn’t reflect the evolutionary tree !

And in the same link, but further down we have the Domains Bacteria, Archaea & Eukarya with the latter divided into the ‘kingdoms’ Excavata, Amoebozoa, Opisthokonta, Rhizaria, Chromalveolata & Archaeplastida

Micheal: Yeah, Chromista and Protozoa (as a Kingdom) are used in the paper, as I gather, just out of historical necessity. Those are how the groups are split in the source database. Two of the authors are protistologists well versed in the newer more up to date 5/6-Kingdom (or super group) Eukaryote taxonomies (EXcavata, etc).

Not that I know of. There are lots of larger scope papers on the subject though, but no layman level books that I can think of. And the wikipedia write-ups, while mostly really good, tend to lean towards Cavalier-Smith’s taxonomy, which can often times be controversial.

IMO Another area of serious underestimate in marine life that I doubt is captured is in the ecology of sea mounts. I read there’s maybe 100,000 sea mounts each one with wide environmental gradations due to the vertical geometry ~ that’s an awful lot of ‘islands’ that could drive speciation. As an example it would not surprise me if one widely distributed apparently single species of kelp found along a mount range were actually 100s of species. We simply don’t have a grasp on the ecology of the seas.

Why is Fig. 2 plotted on log-log coordinates and the power curve fit to the data? If we’re comparing predicted to observed, surely a linear relationship is the benchmark, and indeed the exponent here is 1.03, indicating linearity even on arithmetic coordinates. I smell r-squared inflation.

Also, while I’m not sure the specific model is critical, but Cosma Shalizi criticizes the overuse of power laws and hence implicitly at least its physical or biological “exciting” models (self-similarity, criticality, Lèvy flight).

Shalizi et al derived a test for power laws, and found that many claimed such isn’t. One can often use exponential fits, say.

Under pretty much any system of classification, there are large elements of convention in the ranking of taxa. Many classifications are consistent with a given tree, and it’s perfectly plausible to have a classification with 30 animal phyla or 15. With classes, orders, and families the range of numbers increases greatly. So, my initial reaction to this is that it can’t be interesting (or correct), since it’s based on a conventional system of names, and the criteria of the conventions varies among taxa and workers. Figure G, however, is intriguingly linear, but I wonder if it would hold up if we added in tribes, suborders, etc. (because there is no natural ordinal scale of taxonomic rank), or if it’s just one of those things where if you take the log, damn near everything is linear.

Exactly my thoughts. The Linnaean levels are subjectively placed, so that they don’t represent a certain amount of diversity, disparity, unique evolutionary history, etc. Orders can have a single species, or hundreds or thousands of species for instance. Add to that the fact that in many groups of organisms, ranks are still often para- and polyphyletic so don’t even represent real entities, and I can’t put much stake in this study.

Different ideas about the correct classification of species into a taxonomic hierarchy may distort the shape of the relationships we describe here. However, an assessment of the taxonomic hierarchy shows a consistent pattern; we found that at any taxonomic rank, the diversity of subordinate taxa is concentrated within a few groups with a long tail of low-diversity groups (Figure 3P–3T). Although we cannot refute the possibility of arbitrary decisions in the classification of some taxa, the consistent patterns in Figure 3P-3T imply that these decisions do not obscure the robust underlying relationship between taxonomic levels.

Having recently read Carl Zimmer’s “A Planet of Viruses” it has become clear to me that the whole biosphere is merely a support system for the billions of viruses which are the true masters of Earth. I know viruses are not generally thought of as alive but no parasite can complete its life cycle without its host so the distinction is a bit arbitrary. So the three largest groups of life forms on the plant, viruses, archea and bacteria are the ones we know least about.

OTOH I find it interesting that 89% of described species are animals. Is there any explanation for this?

I am struck in particular by how much less speciose plants and especially the protists are (as compared to the animals). I would have predicted that especially the protists would have been far more numerous than the animal species, given that their small size might give rise to many more specialized environments and more opportunities for allopatric speciation (sexual or otherwise).

I suppose there are good reasons why plants and protists are so much less diverse?

For one thing, according to my understanding most classified ‘protists’–at least the microscopic unicellular kind–have worldwide distributions or nearly so. You don’t see the kind of biogeographical patterns as with animals or plants. I’d guess, though, that classification of these organisms has been by microscopic morphology, and that invisible patterns of genetic diversity including local endemism, etc. might be obscured.

The biogeography stuff is starting to change, some people (very few) are looking at it for microbial eularyotes. Mostly it is just something that hasn’t been investigated very well.

Traditionally species were described based on morphology (which can be problematic when many of them have multiple cell morphologies and life stages, flagellated versus non-flagellated for instance). That is still a requirement for species description and deposition in these sorts of databases. But there is a hell of a lot of sequence data out there, we probably know a lot more about distributions and populations from that data but much of it doesn’t fit neatly into the rigorous (and now archaic) requirements for strict species descriptions.

Keep in mind that the definitions of a species in Plants and among the protists are very different than what they are in Animals.

With regards to the protists there can be a much higher level of genetic variation and niche adaptation without necessarily being deemed a new species (may instead be classified as a sub-type or strain). In addition microbial taxa are generally less catalogued and studied than big multi-cellular organisms.

Perhaps nematodes are under counted. There is an argument in an old textbook that if all other multicellular eukaryotes were removed, but nematodes left in place, we would see ghostly images, made up of nematodes, of all the removed eukaryotes. If there are specific nematodes parasitic on each non nematode species, and restricted to those species, then, with all the more generalized and free living nematodes, there should be more species of nematodes than all other multicellular eukaryotes combined.

I think identifying nematode species morphological is not easy. Hopefully molecular methods will reveal the true extent of nematode species diversity.

“In short, if all the matter in the universe except the
nematodes were swept away, our world would still be dimly
recognizable, and if, as disembodied spirits, we could then
investigate it, we should find its mountains, hills, vales,
rivers, lakes, and oceans represented by a film of
nematodes. The location of towns would be decipherable,
since for every massing of human beings there would be a
corresponding massing of certain nematodes. Trees would
still stand in ghostly rows representing our streets and
highways. The location of the various plants and animals
would still be decipherable, and, had we sufficient
knowledge, in many cases even their species could be
determined by an examination of their erstwhile nematode
parasites.”
N.A. Cobb, from “Nematodes and Their Relationships”, 1915

Thanks for that! I was just rereading it when the Keeton part jumped out at me and I realized, hey, that was my first undergrad bio text. A few minutes of rummaging in the basement and I found the book and then the right section. (Yes, I’m a fossil.) (Tho my Keeton is the ’67 edition; lest anyone think it dates back to the Cobb quote…)